1. Field of the Invention
The present invention relates in general to implanting ions into a substrate metal and more particularly to improving the corrosion resistance of a substrate metal by implanting ions therein using oxygen in the ion implantation vacuum chamber.
2. Description of the Related Art
In general, the corrosion resistance of various metals such as iron-based alloys, aluminum, and aluminum-based alloys is provided by an oxide film which forms on the surface of the normally active metal and which prevents degradation by the environment. However, in environments that contain anions, such as chloride, the oxide film is locally degraded causing a loss of passivity and localized corrosion.
In the case of aluminum and aluminum alloys, it has been proposed to introduce alloying elements which improve corrosion resistance. However, such alloying elements create intermetallic phases which can disrupt the oxide film. Also, the alloying elements which improve corrosion resistance may be inconsistent with strength considerations. Moreover, most alloying elements which improve corrosion resistance have a limited solubility in aluminum.
It has also been proposed to form a solid solution surface alloy with surface modification techniques. Such surface modification techniques are non-equilibrium techniques, introducing alloying elements to a concentration beyond the normal solubility limits. The surface modification techniques, such as ion implantation, use a directed energy beam. More specifically it has been proposed to implant chromium ions into iron and low-alloy steel thereby producing a surface layer similar to stainless steel. Also, implantation of tantalum ions into low alloy steel was found to produce a surface which was somewhat more corrosion resistant than that for chromium implanted low alloy steel. For aluminum and aluminum alloys, it has been proposed to introduce molybdenum or tantalum atoms to increase the resistance to pitting.
One of the factors which limits the effectiveness of surface modification using ion implantation is sputtering, the erosion of atoms as a result of the impact of the incident ions being implanted. In the initial portion of the ion implantation process, atoms of the substrate metal are sputtered away. However, as the implantation progresses, the concentration of the implanted species increases and the erosion starts to effect the implanted species. Eventually the loss of atoms from sputtering equals the rate of implantation. At this point a steady state is reached. The steady state concentration can be given by 100%/(S+1), where S is the sputter coefficient expressed in atoms removed per incident atom (atoms/ion).
It has been found that when tantalum atoms are implanted into steel at an implantation energy of 150 keV, the sputter coefficient is approximately 9. Therefore, the resulting surface concentration based on the formula 100%/(S+1) is 10 atomic %. The maximum concentration of tantalum is at the surface of the substrate metal (steel in this case), and the tantalum concentration is greater than 5 atomic % to a depth of only 250 xc3x85 (1 microinch). While the tantalum-implanted steel exhibits some improvement in corrosion resistance, the low surface concentration is believed to limit such improvement in corrosion resistance.
As an alternative to using ion implantation, it has been proposed to deposit a coating on the substrate metal, for example using a technique such as physical vapor deposition. However, when a coating is deposited on the substrate metal by physical vapor deposition, there are potential adhesion problems between the substrate metal and the coating. Also, pinholes may form in the coating, which could lead to accelerated corrosion. A second method of forming a surface layer is ion beam-assisted deposition (IBAD). According to this method, a layer is deposited on the surface of the substrate metal. Then, an ion beam is directed to the layer at an implantation energy which is significantly less than that used for standard ion implantation. It has been suggested that IBAD coatings should be greater than 0.5 microns in thickness in order to cover defects in the modified layer. Thus, coatings and IBAD have inherent drawbacks when used as a means of preventing corrosion.
Accordingly, it is an object of the present invention to provide a method of reducing surface corrosion in metals which does not involve conventional alloying.
Another object of the present invention is to produce a corrosion-resistant metal which does not involve coating the substrate metal.
It is yet a further object of the present invention to provide a method of ion implantation in which fewer implanted atoms are eroded by sputtering.
According to the present invention, a metal, such as iron, steel, aluminum and an aluminum alloy, are placed in an ion implantation vacuum chamber. Oxygen is then introduced into the ion implantation vacuum chamber to a pressure in the range of 1xc3x9710xe2x88x925 torr to 10xc3x9710xe2x88x925 torr. Then, a beam of transition ions, such as tantalum ions, are directed at the surface of the substrate metal.
The metallic surface layer of the present invention has transition metal atoms integrated with substrate metal atoms. The substrate metal has a transition metal concentration of at least 5 atomic % to a depth of over 250 xc3x85.